The impact of aerodynamic flow on the performance of an airborne optical system is becoming a critical issue in the development and engineering of IR-electrooptic (EO) systems. The analysis of this impact is now at the forefront of IR-EO system research, and a significant effort has been made on this issue in recent years. For a good exemplary overview of the topic, see G. W. Sutton, “Aero-optical Foundations and Applications”, AIAA Journal, vol. 25, No. 10, October 1985, p. 1525.
FIG. 1 illustrates the problem addressed by the present invention. Specifically, FIG. 1 shows a navigation pod 10 suspended below a wing 11 of an aircraft flying at supersonic speed. The front of pod 10 is a transparent dome 12. Mounted within pod 10 is an EO system 14 that captures and processes images of the surrounding environment ahead of pod 10. (The portion of EO system 14 that is invisible from outside pod 10 is shown in phantom.) In supersonic flight of aerodynamic bodies such as pod 10, the air surrounding dome 12 is heated and compressed significantly, and the flow becomes turbulent. These effects cause changes in the local index of refraction of the medium or “flow field” 16 surrounding EO system 14, and lead to optical aberrations that affect detector performance, and with it, the performance of pod 10. These aberrations include a mean shift in image position (“boresight error”) together with mean blur that is represented as a mean field modulation transfer function MTFMF, and turbulence related effects, mainly image spread blur represented as a turbulence modulation transfer function MTFt. All three of these aberrations have to be calculated so that they can be taken into account accurately in system engineering and possibly removed through compensating measures.
Heretofore, optical aberrations in a flow field have been computed by using a computational fluid dynamics (CFD) program to compute the corresponding density field, followed by conversion of the density field to an index-of-refraction field. Then, using the “thin film” approximation, the index of refraction is integrated along a set of straight parallel rays through the flow field to obtain propagation phase differences along these rays. These phase differences indicate the extent to which an initially plane wave is distorted (blur) and tilted (boresight error) by propagation through the flow field. In practice, however, the use of an approximation that assumes straight parallel rays produces estimates of the optical aberration that are insufficiently accurate.
Ray tracing through aninhomogeneous optical medium is well-known in the art, and is used, for example, in the design of lenses. In principle, ray tracing code could be included in a CFD program to provide numerically accurate estimates of the optical aberrations. This, however, would require great skill on the part of the programmer, who would have to be expert in computational optics to be able to anticipate and deal with the various numerical instabilities that would arise during the ray tracing calculations, as is known in the art.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method for calculating and compensating for optical disturbances occurring in a supersonic flow field that does not use embedded ray-tracing in a CFD program, does not use a thin-screen approximation, and therefore does not suffer from the prior art disadvantages listed above.